ON THE COSMOLOGICAL DOMAIN-WALL PROBLEM FOR THE MINIMALLY EXTENDED SUPERSYMMETRIC STANDARD MODEL

We study the cosmology of the Supersymmetric Standard Model augmented by a gauge singlet to solve the mu-problem and describe the evolution of the domain walls which are created during electroweak symmetry brea king due to the discrete Z(3) symmetry in this model. The usual assump tion that (gravitationally induced) non-renormalisable terms which exp licitly break this symmetry may cause the walls to collapse on a cosmo logically safe timescale, is reconsidered. Such terms are constrained by considerations of primordial nucleosynthesis, and also because (by not respecting the Z(3) symmetry) they induce divergences which destab ilise the hierarchy and reintroduce the mu-problem. We find that, even when the Kahler potential is 'non-minimal' (i.e. when the hidden sect or couples directly to the visible), the model is either ruled out cos mologically or suffers from a naturalness problem.